I would guess they havn't fully screwed the heatsink assembly back onto the cpu properly and its not making full contact causing a serious hot spot. This would make the laptop think its cooking and thus put the fan on full speed. I may be completly wrong but thats my thoughts. If there is a flap underneath for cpu inspection it is worth taking it off and checking the screws to make sure they are fully tightened.

Hope this helps.

Regards.

Adam.